Hi Percy!
It's hard to tell if you're an alcoholic, but your routine is a bit abnormal for sure. You are capable of going 6 days a week without any alcohol, so that's normal. But on the day you drink, you down 1.5 litres of wine and several pints of beer. That is quite a binge. I mean, if it was your 40th birthday, I could understand but... once a week?
My second thought is that you're looking for lower alcohol alternatives when going out. It's still alcohol. And, don't we all drink to get drunk? Will a 2% shandy satisfy that craving? Or will it lead to a guinness and then a bottle. I ask because for me, I hate alcohol. I hate the taste. I have no interest in drinking a beer with friends. I want to get a good, hard buzz when I drink. Or why bother?
All that aside, in my opinion if you decide to have a party night once a week, and consume all your weekly alcohol on one night, you're probably doing your body harm by concentrating it all in one night. You're going to tax your liver and kidneys and you'll be dehydrated the next day. With hangover (I'm sure?)
I wish you luck on your journey to sobriety. If you intend to still have your party night, I highly suggest you stay hydrated, pace yourself, consume lower alcohol beverages, and have a designated driver. One of the biggest health risks in drinking heavily is the decisions one makes while drunk which can lead to serious consequences, including death (accidental or otherwise).